Many modern electronic systems utilize a PFC circuit as part of a power supply circuit. The PFC circuit is typically utilized to generate the bulk voltage for the rest of the system, and to shape the current waveform so that it tracks the incoming line voltage. By forcing the current waveform to match the incoming line voltage waveform, the load the system presents to the line looks resistive and the power factor is near 1.0. The PFC circuit conventionally does this by applying PWM (pulse width modulation) control to a power FET that is in series with a boost inductor across the line.
A schematic of such a conventional PFC circuit 100 is illustrated in prior art FIG. 1. The PFC circuit 100 includes input Vin 120, diodes D1-D4 130, inductor 140, FET Q1 150 having associated charge capacitor C1, diodes D5 and D6, and capacitor Co 170 across which output terminals 180 are disposed.
FIG. 2 illustrates the gate driver signal 210 typically applied to the FET 150, Vds of the FET 220, PFC boost inductor current 230, and the input voltage Vac 240. At light load, the PFC goes to discontinuous conduction mode, once the boost current declines to zero, and the boost inductor 140 will resonate with PFC FET Q1's parasitic capacitance C1. The resonant current becomes so significant that it distorts the AC current waveform. The resonant current contributes to total AC current, adding in one switching cycle, and may subtract in the next switching cycle, which causes large current steps. The current resonates between boost inductor and C1, causing a negative step in the boost inductor current, which may be seen at point 260, for example. This results in an increased THD (total harmonic distortion) in the PFC circuit. However, modern needs in PFC circuits are requiring lower THD.
As illustrated in FIGS. 3a and 3b, conventional PFC circuits are hard switching. The PFC FET turns on randomly in respect to resonant current's phase when boost current becomes discontinuous. There are chances that the FET may turn on at a high Vds voltage and result in significant switching loss. The above problems leave a need for a PFC circuit that can provide zero voltage and zero current switching to improve efficiency.